Pipe system for receiving and transporting lime sludge from a white liquor filter

ABSTRACT

The pipe system is for receiving and transporting lime sludge from a filter medium in a continuously operating pressurized filter  1  in a causticizing process, where lime sludge is scraped off from the filter medium  2  and falls down into receiving chutes  4   a  for the lime sludge. Conventional storage tanks for scraped-off lime sludge and their mechanical agitators are thus replaced by a pipe system  11  where the pipe system does not have a mechanical agitator at all but has a flow cross section which maintains a flow rate in the lime sludge so that sedimentation of the lime sludge does not take place from the receiving chute  4   a  to the feed-out opening  30  of the pipe system.

PRIOR APPLICATION

This application is a U.S. national phase application based onInternational Application No. PCT/SE02/00195, filed 5 Feb. 2002,claiming priority from Swedish Patent Application No. 0100411-8, filed 7Feb. 2001.

TECHNICAL FIELD

The invention relates to a pipe system for receiving and transportinglime sludge and mud from a filter medium in a continuously operatingfilter in a causticizing process, in which white liquor is separatedfrom the lime sludge.

STATE OF THE ART

The green liquor entering the causticizing process consistsfundamentally of sodium carbonate. In the process, the green liquor ismade to react with burnt lime, calcium carbonate (lime sludge) andsodium hydroxide (white liquor) being formed. The white liquor isfiltered off in tube filters or disc filters, the latter in particularusually being of the pressurized type, after which the white liquor isreused in the sulphate process for cooking pulp wood chips. The limesludge obtained from filtration is burnt so as to form burnt lime again.

Modern effective filtration arrangements for white liquor filtrationproduce lime sludge/filter cakes with relatively high dry contents butin which the lime sludge cakes, after they have been removed from thefilter medium, constitute a material with highly adhesive properties.The material of the lime sludge cakes therefore has a tendency to adhereto the inner walls in the chute(s) via which it is transported away fromthe filter medium. This can give rise to acute operational stoppages orstoppages for cleaning on account of clogging, which reduces thecapacity of the filtration arrangements and can cause problems and leadto other disadvantages.

The cleaning of ducts through which adhesive substances are transportedcan be effected by, for example, manual cleaning or automatic cleaning.Manual cleaning has many obvious drawbacks such as operational stoppagesor partial operational stoppages and a poor working environment.Automatic cleaning can be performed using well-known methods such as,for example, with brushes, beating tools, ultrasound, steam-jetting, andliquid application where this is possible.

In continuously operating filtration arrangements for liquid suspensionssuch as, for example, those described in SE-C-463771 (=U.S. Pat. No.4,929,355), a filter cake in the form of lime sludge is formed on afilter medium. This lime sludge is scraped off by doctors (knives) anddrops down into receiving chutes, in which further dilution by dilutionliquid is essential for it to be possible to transport the lime sludgeonward. In said SE-C-463771, a feed-out screw is shown directly afterthe feed-out chutes in one embodiment, which screw feeds onward to afeed-out valve arrangement. In another embodiment shown, thisarrangement is illustrated in the form of a level-regulated storagetank, in which there is a mechanical agitator. Some embodiments of thisprocess comprise a sloping collecting pipe after the feed-out chutes,which collecting pipe leads to the storage tank. In order to avoidsedimentation in the sloping pipe, the lime sludge slurry is circulatedfrom the bottom of the storage tank to the highest point of the slopingcollecting pipe.

A corresponding solution with a storage tank and mechanical mixer isdisclosed in U.S. Pat. No. 5,151,176. In other applications with tubefilters as well (see U.S. Pat. No. 4,264,445), use is made of a storagetank and a mechanical mixer in order for it to be possible to deal withthe filter cake taken out of a pressurized tube filter. The bulkystorage tank and the mechanical agitator arranged therein wereconsidered necessary in order for it to be possible to keep the limesludge well mixed with added dilution liquid and to preventsedimentation of the lime sludge.

The known art has led to handling systems for the lime sludge beingunnecessarily expensive and space-consuming. Conventionally employedtreatment vessels with mechanical agitators for receiving the limesludge resulted in considerable costs. In the case of application in awhite liquor filter with a relatively low capacity of 2,500 m³/24 hrs,and a vessel with dimensions of 1900×2800 (diameter×height), the costcan be in the order of SEK 800,000 (approximately US$ 100,000). In awhite liquor filter with a higher capacity of 8,000 m³/24 hrs, and avessel with dimensions of 2800×3300 (diameter×height), the cost can bein the order of SEK 1,500,000 (approximately US$ 200,000).

Another disadvantage is that the mechanical agitators, in particularwhen applied in pressurized white liquor filters, have to be providedwith packing boxes, which often lead to leakage problems.

The space requirement for storage tanks also limits the possibilities ofstructural alterations in existing installations where available spacedoes not always allow the storage tank to be positioned adjacent to thefilter, or results in the filter and the storage tank having to belocated on different floors in the installation. In most cases, thestorage tanks used in white liquor filtration have been virtually thesame size as the white liquor filter itself, which is in itself veryspace-consuming.

BRIEF DESCRIPTION OF THE INVENTION

One object of the invention is to provide a system for receiving andtransporting lime sludge from a filter medium in a continuouslyoperating pressurized filter in a causticizing process, in which whiteliquor is separated from the lime sludge, and in which the system can beminimized in terms of size and cost at the same time as theconventionally used mechanical agitators can be dispensed with entirely,notwithstanding the strong tendency of the lime sludge to adhere to thewalls in the handling system.

Another object is to obtain a system in which the dry content can bekept high in the system, which high dry content has not been consideredpossible in the large storage tanks as the lime sludge is to be kept ina uniform solution throughout the storage tank. The total quantities tobe handled are then smaller, which results in considerable savings onpipeline systems and associated equipment.

Another object is to permit structural alterations in existinginstallations in which the available space is limited, it being possiblethen to arrange high-performance filter equipment in a small availablespace.

In a preferred embodiment, the risk of sedimentation in the system isreduced by using forced recirculation which affords an increased speedthroughout the volume of the lime sludge mixture received. In anadvantageous embodiment, a modified recirculation principle can be usedin order to keep all the receiving chutes free from accumulations oflime sludge without having to dilute the lime sludge further withdilution liquid. In this embodiment, the storage tank can be minimizedor eliminated completely.

Further characteristics, aspects and advantages of the invention emergefrom the patent claims below and from the following description of anumber of possible embodiments.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a first variant of the white liquor filter according to theinvention;

FIG. 2 shows a side view of the white liquor filter in FIG. 1;

FIG. 3 shows a second variant of the invention, and

FIG. 4 shows a side view of the white liquor filter in FIG. 3.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

A pressurized white liquid filter according to the invention is shown inFIG. 1 and in cross section in FIG. 2. The pressurized filter vessel 1is fed with unfiltered liquid in the form of white liquor mixed withlime sludge via the inlet 8. The unfiltered liquid forms a liquid levelinside the filter which reaches a level just below the shaft 5 of thefilter, and not above the inlet opening of a receiving chute for thelime sludge, which is described below.

Arranged on the shaft is a number of disc-shaped filter elements 2 whichare each covered with a filter medium, in most cases in the form of afine-meshed cloth. The filter consists of at least two discs coveredwith filter medium and arranged on a hollow rotating shaft. In certainapplications, as many as fourteen discs may be used. FIG. 1 shows 3discs, one of which is concealed. White liquor is separated from thelime sludge by withdrawal through the filter medium of the disc filterelement and is conveyed out of the filter via the hollow shaft, eachdisc filter element comprising stripping means interacting with thefilter medium for freeing lime sludge accumulated on the filter medium,and a receiving chute being arranged with a receiving opening under eachstripping means, and between two discs, so that each receiving openingcollects lime sludge from two filter surfaces.

The white liquor is separated during filtration through the filtermedium and is conducted via the inside of the filter element down to thehollow shaft 5 and onward to the gas separation vessel 6. The hollowshaft 5 can preferably be divided into sections, so that the hollowspace in the shaft is made up of cake-slice-shaped spaces, seen in crosssection through the axis.

A blower 7 is connected to the top of the vessel 6 and conducts gas backto the filter vessel 1 for pressurization of the process. Agitation (notshown) of unfiltered liquid also takes place in the filter for thepurpose of preventing sedimentation. The agitation can be effected bymeans of, for example, blowing in air or liquid, and with or withoutejectors.

The shaft 5 is rotated continuously by a motor, a deposit of lime sludgebeing built up gradually on the filter medium as the white liquor passesthrough the filter medium. The filter normally operates with a precoatlayer which is maintained intact during the majority of the operatingtime. However, the invention also functions on filters which operatewithout a precoat covering.

The lime sludge, which is deposited on the precoat or directly on thefilter medium, is scraped off by a knife 3 (doctor) which is arrangedabove the level of the unfiltered liquid, and it then drops down into areceiving chute 4 which likewise has its opening above the level of theunfiltered liquid. Located in the receiving chute are nozzles fordilution liquid 10, which dilute the stripped dry lime sludge to a drycontent of at least 15%, preferably more than 20%, and typically in therange 25-30%. Dilution makes it possible to counteract to a certainextent clogging of the receiving chute.

In conventional white liquor filters, the receiving chute opens, via asloping collecting pipe, in a storage vessel of considerable volume, inwhich a mechanical agitator is also arranged, in most cases in the formof a motor-driven propeller, which is driven continuously so as to avoidsedimentation and therefore keep the lime sludge in solution. In theseconventional systems as well, a certain amount of the lime sludge beingmechanically agitated is recirculated to the collecting pipe in orderthat the latter does not become clogged by sedimented lime sludge.

In accordance with the invention, the storage vessel receiving the limesludge is thus replaced by a pipe system 11 which is common to thereceiving chutes 4 and has a flow cross section which maintains a goodflow rate and in which there is no need for mechanical agitators. Eachreceiving chute has a flow cross section δ₁, hereinafter referred to asthe first flow cross section. The pipe system is arranged below thefilter vessel 1 and connected to the respective receiving chutes, sothat these chutes lead to the lime sludge dropping down verticallythrough the receiving chutes, without deflections and with contact withthe walls in the chute being minimized, and on down to the pipe system11. The connecting portion of the pipe system, the collecting pipe, towhich the receiving chutes 4 are connected, is arranged essentiallyparallel to the shaft 5 of the filter, but with its downstream end, seenin the feed-out direction of the lime sludge, slightly lower than itsupstream end. This connecting portion of the pipe system preferably hasa circular cross section with a flow cross section δ₂, hereinafterreferred to as the second flow cross section. By virtue of this design,a natural drop for the collected lime sludge out towards the outlet isobtained.

The pipe system is designed in such a manner that, from the connectionof the receiving chutes to the feed-out opening 30 of the pipe system,it has a flow cross section which maintains a good flow rate in the limesludge, so that sedimentation of the lime sludge does not occur.

In the embodiment shown, the connecting portion in the pipe system forthe receiving chutes has the same diameter d₂ over its extent, butvariants with gradually increasing diameter can be used as, from eachnew receiving chute connected, seen in the feed-out direction of thelime sludge, a flow of diluted lime sludge is added. In this way, a goodhigh flow rate can be maintained, and sedimentation can be avoided.

The connecting portion of the pipe system, which preferably has acircular flow cross section δ₂, ends in a small downpipe 14 at itsdownstream end. The downpipe is suitably a vertically upright downpipewith a local increase in the flow area in the pipe system in thedownpipe corresponding to the flow cross section δ₃. However, the flowcross sections of the pipe system in other parts, which other partsconstitute at least 75%, preferably at least 90%, of the total length ofthe pipe system between its inlet and outlet, are preferably designed insuch a manner that these other parts have a flow cross section whichdoes not exceed the flow cross section δ₂. The total length of thedownpipe, L₁, in the vertical direction is:

L ₁≧2.0·d ₂ and L ₁≧1.5·d ₃.

In a suitable embodiment of a relatively large pressurized disc filterwith fourteen discs of a diameter of 3 m, where the maximum flow of limesludge is 0.095 m³/s, the collecting pipe has a diameter of roughly 700mm, the downpipe has a diameter of roughly 900 mm, and the outlet has adiameter of roughly 200 mm. In the case of smaller filters with a lowercapacity, in particular the diameter of the downpipe can be reducedfurther according to the table below:

Number of discs Ø = 3 m 6 8 10 12 14 Max. 0.041 0.054 0.068 0.081 0.095flow lime sludge (m³/s) Downpipe 600 700 800 900 900 Ø (mm)

The height of the downpipe is suitably greater than its diameter,appropriately between 1800 and 2200 mm for a 900 mm diameter, and 1000and 1500 for a 600 mm diameter. When smaller filters are used, withsmaller diameters, the collecting pipe and the outlet pipe can of coursebe made smaller, but with essentially the same proportions as in thecase with 3 m discs.

However, the pipe system does not have to have a circular cross sectionin all its parts, but can also have an oval or rectangular crosssection.

In order to avoid the lime sludge sedimenting in the outlet from thedownpipe, the lower part of the downpipe is designed with an essentiallycontinuous conical transition from the third flow cross section δ₃ ofthe downpipe to the fourth flow cross section δ₄ in the feed-out openingwhich follows for the lime sludge received.

A level meter 21 is suitably connected to the downpipe, which meter isin turn coupled to a regulating unit 22 which is finally connected tothe regulating valve 20. In this way, the level of the lime sludge canbe regulated depending on the signal from the level meter, oralternatively with regulation by an adjustable pump. The pump can beadjusted by, for example, either displacement regulation or speedregulation.

The valve 22, or alternatively a pump (not shown), forms acounterpressure-generating means t in the feed-out opening 30 from thefilter for the purpose of maintaining the pressure in the filter.

In order to maintain a uniform and continuous flow in a minimized systemwithout mechanical agitators, the relationship between the third flowcross section δ₃ in the downpipe and the second flow cross section δ₂ ofthe collecting pipe is:

δ₃≦4.5·δ₂ and preferably δ₃≦4·δ₂.

At the same time, the relationship between the fourth flow cross sectionδ₄ in the outlet and the second flow cross section δ₂ is:

δ₄≦0.3·δ₂ and preferably δ₄≦0.2·δ₂ ²·δ₂.

In order to reduce the risk of sedimentation in the pipe system further,a conventional recirculation system 12 is connected to the pipe system,the inlet 12 a of the recirculation system being arranged after theconnection of the last chute to the pipe system, seen in the feed-outdirection of the lime sludge through the pipe system, and an outlet 12 bbeing arranged in front of the connection of the first chute to the pipesystem. A pump 12 c is arranged in the recirculation system forrecirculation of a part quantity of diluted lime sludge from its inletto its outlet. The inlet 12 a of the recirculation system is suitablyarranged at the lowest point of the pipe system, which is after thelevel measuring tank 14 in the embodiment shown.

The pump 12 c, preferably a sludge pump resistant to wearing material,is preferably driven continuously at constant speed during the majorityof the operating time of the filter, the diluted lime sludge being keptin boosted circulation so that sedimentation is counteracted moreeffectively.

FIG. 3 shows a second variant of a pressurized white liquor filteraccording to the invention in cross section. Those components of thefilter which have the same function as those shown in FIGS. 1 and 2 havethe same reference numbers, and are not described further. In thisvariant, the pipe system is adapted so that thecounterpressure-generating means in the pipe system feed-out opening 30b of the pressure filter instead consists of a rising pipe which opensat a level above the level of the unfiltered white liquor mixed withlime sludge in the filter. By regulating the pressure in the filter, itis possible to maintain the desired level of lime sludge in thereceiving chutes 4 a. The level measuring tank 14 from FIG. 1 is thusomitted. In FIG. 3, the level is instead measured in the receivingchutes 4 a via a level meter 21 b, the signal A from which is sent to aregulating unit 22 b. The regulating unit acts on valves 26 a/26 b, bywhich the overpressure in the filter and thus the filtration flowthrough the filter can be regulated. The level 30 b at which the limesludge can be fed out in the variant with a rising pipe corresponds to araised level relative to the level of the unfiltered liquid in thefilter. The maximum raised level corresponds to the maximum pressurelevel inside the filter minus the pressure losses in the pipe system.

A modified recirculation system 12 in FIG. 3 is connected to the pipesystem, the inlet 12 a of the recirculation system being arranged afterthe connection of the last chute to the pipe system, seen in thefeed-out direction of the lime sludge through the pipe system, and anoutlet 12 b being arranged in front of the connection of the first chuteto the pipe system. A pump 12 c is arranged in the recirculation systemfor recirculation of a part quantity of diluted lime sludge from itsinlet to its outlet. The inlet 12 a of the recirculation system isarranged at the lowest point of the pipe system, which is after thelevel measuring tank 14 in the embodiment shown. In contrast to therecirculation system shown in FIG. 1, at least one second outlet 12 e isalso arranged so as to open in each receiving chute 4 a at a levelbetween its receiving opening 4 and its connection to the pipe system.Each outlet 12 e is provided with recirculated diluted lime sludge viathe branch 12 from the pressure side of the pump 12 c. In this way, aboosted flow is obtained through the receiving chutes as well, as aresult of which the risks of sedimentation and clogging in the receivingchutes are also reduced.

The invention can be modified in a number of ways within the scope ofthe accompanying patent claims, among which the following modificationsmay be considered.

For example, the recirculation system may comprise only the outlet 12 ein each receiving chute, the outlet 12 b being omitted. Control of thedesired quantities in each outlet is preferably effected by suitabledimensioning of the cross section of the flow ducts in the recirculationsystem.

The most upstream receiving chute, seen in the feed-out direction of thelime sludge, or the outlet 12 b, can also be provided with a flow ductof maximum cross section, so that a strong basic flow of recirculatedlime sludge is developed at the start of the pipe system.

In the variant with a rising pipe 24, the inlet 12 c can be connecteddirectly below this rising pipe, as some sedimentation of the limesludge can take place during onward transport upwards in the rising pipe24.

The recirculation system shown can also be supplemented with air orwater, and also air/water mixtures, being intermixed or substituted.

The circulation flow of lime sludge does not necessarily have to betaken from the lowest point in the system either, although this ispreferable.

In the variant shown in FIG. 3, the rising pipe 24 can be replaced by aregulating valve (or pump) in the same manner as in FIG. 1.

Depending on the nature of following systems, the level regulation inFIG. 3 with pressure regulation 26 a/26 b can also be used in the systemshown in FIG. 1.

What is claimed is:
 1. Pipe system for receiving and transporting limesludge from a filter medium in a continuously operating pressurizedfilter in a causticizing process, where the filter comprises at leastone filter element covered with filter medium and partially immersed inwhite liquor mixed with lime sludge, and where white liquor is separatedfrom the lime sludge by withdrawal through the filter medium of thefilter element, each filter element comprising stripping meansinteracting with the filter medium for freeing lime sludge accumulatedon the filter medium, and where at least one receiving chute with afirst flow cross section δ₁ is arranged with a receiving opening undereach stripping means and above the level of the white liquor mixed withlime sludge for receiving stripped lime sludge, which receiving chutecomprises an arrangement for supplying dilution liquid to the limesludge to obtain a lime sludge concentration exceeding 15%, preferablymore than 20%, characterized in that each receiving chute is connectedto a pipe system with a second flow cross section δ₂, which system iscommon to the chutes, and in which the pipe system, from the connectionof the receiving chutes to the feed-out opening of the pipe system, hasa flow cross section which does not exceed a third flow cross section δ₃and which maintains the flow rate in the lime sludge so thatsedimentation of the lime sludge does not take place, in which the pipesystem, seen in the flow direction of the filtered lime sludge, from theconnection of the receiving chutes to a feed-out opening of the pipesystem, does not have a mechanical agitator, and in which the feed-outopening has a fourth flow cross section δ₄ which constitutes thesmallest flow cross section of the first, second, third and fourth flowcross sections, and in that a counterpressure-generating means isarranged in the feed-out opening for the purpose of maintaining thepressure in the filter.
 2. Pipe system according to claim 1,characterized in that the relationship between the third flow crosssection δ₃ and the second flow cross section δ₂ is: δ₃≦4.5·δ₂ andpreferably δ₃≦4.0·δ₂.
 3. Pipe system according to claim 1, characterizedin that the relationship between the fourth flow cross section δ₄ andthe second flow cross section δ₂ is: δ₄≦0.3·δ₂ and preferably δ₄≦0.2·δ₂.4. Pipe system according to claim 1, characterized in that thecounterpressure-generating means is an essentially vertically arrangedrising pipe which opens at a level above the level of the accumulatedlime sludge in the filter.
 5. Pipe system according to claim 4,characterized in that a level meter is arranged in at least onereceiving chute.
 6. Pipe system according to claim 5, characterized inthat the level meter is connected to regulating means for regulating thepressurization in the filter.
 7. Pipe system according to claim 4,characterized in that a level meter (21) is arranged in the pipe system(11) in an essentially vertically upright downpipe with a local increasein the flow area in the downpipe corresponding to the flow cross sectionδ₃, and in that the flow cross section of the pipe system in otherparts, which other parts constitute at least 75% of the total length ofthe pipe system between its inlet and outlet (30), and also in theseother parts has a flow cross section which does not exceed the flowcross section δ₂.
 8. Pipe system according to claim 7, characterized inthat the length L₁ of the downpipe, in the vertical direction, is:L₁≧2.0·d₂ and L₁≧1.5·d₃, where d₂ is the maximum diameter in the pipesystem where the flow cross section does not exceed δ₂, and d₃ is themaximum diameter in the pipe system where the flow cross section doesnot exceed δ₃.
 9. Pipe system according to claim 7, characterized inthat the lower part of the downpipe changes from the third flow crosssection δ₃ to the fourth flow cross section δ₄ in the feed-out openingvia an essentially continuous conical transition.
 10. Pipe systemaccording to claim 7, characterized in that the level meter (21) isconnected to regulating means (22) for regulating the adjustablecounterpressure-generating outlet valve (20).
 11. Pipe system accordingto claim 1, characterized in that a recirculation system is connected tothe pipe system (11, 14), the inlet (12 a) of the recirculation systembeing arranged after the connection of the last receiving chute to thepipe system, seen in the feed-out direction of the lime sludge throughthe pipe system, and an outlet (12 b) being arranged in front of theconnection of the first receiving chute to the pipe system, and a pump(12 c) being arranged in the recirculation system for recirculation of apart quantity of diluted lime sludge from the inlet of the recirculationsystem to its outlet.
 12. Pipe system according to claim 11,characterized in that at least one of water, gas or compressed air isadded to the recirculation of diluted lime sludge.
 13. Pipe systemaccording to claim 11, characterized in that at least one second outlet(12 e) is arranged so as to open in each receiving chute at a levelbetween its receiving opening and its connection to the pipe system. 14.Pipe system according to claim 11, characterized in that the inlet ofthe recirculation system is arranged at the lowest point of the pipesystem.
 15. Pipe system according to claim 1, characterized in that thecounterpressure-generating means is an adjustable outlet valve (20) oran adjustable pump.
 16. Pipe system according to claim 1, characterizedin that the filter consists of at least two disc filters (2) coveredwith filter medium and arranged on a hollow rotating shaft (5), wherewhite liquor is separated from the lime sludge by withdrawal through thefilter medium of the disc filter element and is conveyed out of thefilter via the hollow shaft, each disc filter element comprisingstripping means (3) interacting with the filter medium for freeing limesludge accumulated on the filter medium, and where a receiving chute (4a) is arranged with a receiving opening under each stripping means.